Prior workers have expended significant effort developing ways of representing color information in electronic display systems. FIG. 1 shows one of the earliest color representations, which is commonly referred to as the red, green, blue (“RGB”) color cube representation (hereafter “the RGB cube 10”). Contours of constant color intensity are projected onto the visible exterior faces of the cube. The seven visible vertices correspond to the pure colors of red, green, blue, cyan, magenta, yellow, and white. The hidden [R=0, G=0, B=0] vertex corresponds to black.
The RGB cube 10 evolved from psychophysical experiments supporting the development of color television. The human eye was found to be responsive to RGB hues, so the RGB cube 10 naturally followed. The psychophysical experiments further indicated that the sensitivity of the eye to green was most closely correlated with its sensitivity to black-and-white intensity. Therefore, the National Television Standards Committee adopted a linear transformation from red, green, and blue to an approximate intensity-chrominance-luminance representation. This so-called YIQ representation and/or its related YUV and Y-Cr-Cb variants form the color space of the well-known Discrete Cosine Transform compression algorithms that are employed in the JPEG (JFIF), HDTV, MPEG-2, and MPEG-4 standards.
The Y—Cr-Cb representation is a linear transformation of the red-green-blue representation, in which the luminance term Y is dominated by the color green. This representation has been a staple of broadcast television and video for more than 50 years because it is easy to compute with linear systems, not because it is an accurate representation of intensity, chrominance, and luminance.
Another known color representation is based on the Munsell color classification system. That system specifies color in terms of three values: Munsell hue, Munsell chroma, and Munsell value. The Munsell color system can be visualized as a “tree.” Along the center trunk of the tree is the achromatic value axis, which ranges from black, through shades of gray, to white. Extending out from the trunk are vertical planes, each of which represents a hue. In any given plane, the Munsell chroma value represents the shortest distance of a particular color from the trunk. Thus, the Munsell color system is essentially based on a cylindrical coordinate system. Because none of the Munsell coordinates are independent or uncorrelated with its peers, the Munsell system has limited usefulness for machine vision and image compression/coding.
One alternative color representation is the hue-lightness-saturation (“HLS”) or hue-saturation-brightness (“HSB”) color space. HLS is based on approximate planar geometry including loose, albeit easily computed, notions of lightness and saturation. Unfortunately, HLS is not a particularly accurate color representation, and therefore has limited machine vision and data compression potential.
Another alternative color representation is the International Color Consortium's 1976 CIElab color specification, which can be viewed as a nonlinear transformation of the RGB cube 10 to a luminance (l), red-green (a), and yellow-blue (b) space. This representation, although an international standard, is unacceptable for use in computer vision and lossy compression applications because both the a and b coordinates are necessary to describe a unique hue.
In addition to the human eye, display technology also sets requirements for color representations. Typical computer displays, such as cathode-ray-tubes (“CRTs”) and liquid-crystal displays (“LCDs”), can typically render images having intensity ranges between 100:1 and 400:1, which correspond to dynamic ranges of 40 to 52 decibels (“dB”). Such dynamic ranges require only 6.64 to 8.64 bits to represent intensity faithfully, whereas RGB images are typically represented with 24 bits, which support a 129 dB dynamic range. No current display technology can render half that dynamic range, but because the RGB cube 10 is inefficient, the entire dynamic range is needed to render a rich color palette with high dynamic range.